Four- versus five-day estrous cycles in rats: vaginal cycling and pregnancy.

Five- compared to four-day cycling rats had earlier onsets and prolonged periods of full vaginal cornification at estrus, and similar times of the first appearance of the "pavement" cells that signal the end of estrus. In mating tests, a larger proportion of five- compared to four-day cyclers became either pregnant or progestational after mating to one ejaculation by the male. As groups, four- and five-day cyclers did not differ in measures of male or female sexual behavior during the mating test. The observed differences between four- and five-day cyclers in the progestational response to one ejaculatory series and in cyclic vaginal cytology are consistent with previously reported hormonal differences between four- and five-day cycling female rats.